1. Field of the Invention
The present invention relates to methods for producing fuel cell units and for producing fuel cell stacks. More specifically, the present invention relates to a method for producing a fuel cell unit which is formed by a membrane electrode assembly (MEA) including a solid polymer electrolyte membrane held by a pair of electrodes, the outside of the membrane electrode assembly being held by a pair of separators, and to a method for producing a fuel cell stack which is formed by stacking a plurality of the fuel cell units. In particular, the present invention relates to a technique by which tightening margin at a sealing portion in the membrane electrode assembly may be made constant without being influenced by the non-uniformity in the thickness of the membrane electrode assembly.
2. Description of Related Art
In conventional solid polymer type fuel cell units, a solid polymer electrolyte membrane, which functions as a cation exchange membrane, is sandwiched by a pair of electrodes, and the outside of each of the electrodes is held by a pair of separators.
Generally, a certain number of the fuel cell units having the above-mentioned structure are stacked and used as a fuel cell stack.
FIG. 11 is a diagram showing an enlarged cross-sectional view of main parts of an example of the fuel cell unit.
In the fuel cell unit 1 shown in FIG. 11, a passage 4 for an oxidant gas (for instance, air including oxygen) is provided on a surface of a cathode side separator 3a which is disposed so as to face a cathode 2a. 
On the other hand, a passage 5 for a fuel gas (for instance, hydrogen) is provided on a surface of an anode side separator 3b, which is disposed as to face an anode 2b, and a passage 6 for a cooling medium (for instance, water or ethylene glycol) is provided on the other surface of the anode side separator 3b. 
Since it is necessary that the oxidant gas, the fuel gas, (hereinafter these gases may be abbreviated as “reaction gas(es)”, and the cooling medium be independently passed through the passages 4-6, respectively, the technique used for sealing between each of the passages 4-6 becomes important.
Examples of portions to be sealed include in the vicinity of a communication hole (not shown in the figure) which penetrates through the separators 3a and 3b in order to distribute and supply the reaction gases and the cooling medium to each of the fuel cell units 1, an outer periphery of a membrane electrode assembly 8 formed by a solid polymer electrolyte membrane 7 and the electrodes 2a and 2b disposed so as to sandwich the solid polymer electrolyte membrane 7, an outer periphery of a cooling medium passage of the separators 3a and 3b, and an outer periphery of the both sides of the separators 3a and 3b. 
As a sealing technique used for the fuel cell unit 1 and the fuel cell stack, one is known in which a solid seal 9 made of a soft material having a suitable resilience, such as an organic rubber, is disposed at sealing portions, and a load is applied to the solid seal 9 in a stacking direction (i.e., the longitudinal direction in FIG. 11) to compress the solid seal 9 so that the sealing portions are sealed using the surface pressure generated thereby.
In the above-mentioned technique, the seal compression amount Δh, which is the tightening margin for the solid seal 9, may be defined by the following formula:Δh=Δh1+Δh2  (1)Δh1=hseal −hMEA  (2)
hseal: the height of the solid seal 9;
hMEA: the thickness of the membrane electrode assembly 8; and
Δh2: the compression amount of the membrane electrode assembly 8 when load is applied.
Here, at each stacking surface of the fuel cell stack, it is necessary that the surface pressure, which is sufficient for an appropriate contact in or between the fuel cell unit(s) 1, be applied in order to suppress the increase in the internal resistance or the contact resistance of the fuel cell unit 1.
However, as it is clear from the above formulae of (1) and (2), if the thickness hMEA of each of the membrane electrode assembly 8 is not uniform, the non-uniformity ΔhMEA is directly reflected to the seal compression amount Δh, which is the lightening margin for the solid seal 9.
As shown in the graph of FIG. 12, the seal compression amount Δh may be expressed by the distance between points of intersection, which are present on the threshold value of a surface load F of the membrane electrode assembly 8 required for obtaining the above-mentioned degree of the surface pressure, formed by the surface load curve (expressed by a two-dotted line in the graph) of the membrane electrode assembly 8 having a predetermined thickness hMEA (hereinafter referred to as “a standard thickness”), and by the surface load curve (expressed by a one-dotted line in the graph) of the membrane electrode assembly 8 having a thickness hMEA which is different from the predetermined thickness hMEA by ΔhMEA. Accordingly, if the non-uniformity ΔhMEA is directly reflected on the seal compression amount Δh, the non-uniformity ΔFs of the seal load Fs (expressed by a dashed line in the graph) is also increased.
Also, if the thickness hMEA is not uniform in in-phase directions of the same membrane electrode assembly 8, the seal surface pressure which acts on the solid seal 9, which in turn acts on the sealing portions, and on the separators 3a and 3b and the membrane electrode assembly 8, is also made non-uniform. Accordingly, the power generation performance of the fuel cell may decrease due to the deterioration of the sealing property, and the fuel cell unit 1 may be bent and deformed due to the non-uniformity in the surface load between the fuel cell units 1.
Although the generation of the bent-deformation may be prevented by increasing the thickness of the separators 3a and 3b, the resultant fuel cell is not suitable for mounting on a vehicle, for instance, since the size and the weight of the fuel cell stack are increased.
Other than the technique relating to the solid seal 9 described above, as a sealing technique relating to the fuel cell unit 1 and the fuel cell stack, one is known in which an adhesive, etc., is filled in a sealing portion in a load applied state in the stacking direction and the sealing portion is sealed by using the adhesive strength at boundary surfaces as disclosed in, for example. Japanese Unexamined Patent Application, First Publication No. Hei 7-249417.
However, in the above technique relating to the adhesive seal, there are problems, such as a low reliablity in the durability of the adhesive strength at the boundary surfaces.